config: fix media path at /var/thoughtsync (no env knob)

The media/data location is no longer configurable — DATA_DIR is a fixed
constant (/var/thoughtsync) and THOUGHTSYNC_MEDIA_ROOT is removed, so a
mutable path can't drift from where the volume is mounted. media_root() =
DATA_DIR/media. Both compose files drop THOUGHTSYNC_DATA_DIR and mount the
data volume at /var/thoughtsync (was the contradictory /data). conftest
drops the stale MEDIA_ROOT monkeypatch (create_app never reads DATA_DIR).

Net env surface: THOUGHTSYNC_DATABASE_URL (required) + THOUGHTSYNC_SECRET_KEY
(optional break-glass).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FRgehjoz7Yv8LkUfADxACm

src/thoughtsync/config.py

@@ -8,19 +8,21 @@ class Config:
     """Bootstrap configuration.
 
     For a basic install, ``THOUGHTSYNC_DATABASE_URL`` is the ONLY required env var —
-    every other tunable lives in the DB-backed Settings UI (rule 25). The remaining
-    env vars are optional "break-glass" / bootstrap items:
+    every other tunable lives in the DB-backed Settings UI (rule 25). The only other
+    env var is an optional "break-glass" item:
 
     - ``THOUGHTSYNC_SECRET_KEY`` — optional override for the cookie-signing secret.
       If unset, a key is generated and persisted in the DB (see
       ``thoughtsync.settings.load_or_create_secret_key``), so sessions survive
       restarts with no volume required.
-    - ``THOUGHTSYNC_DATA_DIR`` — optional, defaults to ``/var/thoughtsync``. Only
-      used for uploaded media (M2); irrelevant to a basic text-notes install.
+
+    Uploaded media lives under ``DATA_DIR`` — a fixed, authoritative path
+    (``/var/thoughtsync``), intentionally NOT configurable (a mutable data path only
+    invites breakage). Mount a volume there if you want uploads to persist across
+    container recreation; a text-notes-only install never writes to it.
     """
 
-    DATA_DIR = os.environ.get("THOUGHTSYNC_DATA_DIR", "/var/thoughtsync")
-    MEDIA_ROOT = os.environ.get("THOUGHTSYNC_MEDIA_ROOT", "")
+    DATA_DIR = "/var/thoughtsync"
 
     DATABASE_URL = os.environ.get(
         "THOUGHTSYNC_DATABASE_URL",
@@ -29,7 +31,7 @@ class Config:
 
     @classmethod
     def media_root(cls) -> Path:
-        return Path(cls.MEDIA_ROOT or os.path.join(cls.DATA_DIR, "media"))
+        return Path(cls.DATA_DIR) / "media"
 
     @classmethod
     def secret_key_env(cls) -> str | None: